Published in the June 2015 FREMCO FIBER NEWS NEWSLETTER
It is common knowledge that fiber blowing requires fairly straight ducts with close to full passage. Luckily, many blowing jobs work fine, but technicians often face problems with the buried or relined ducts. Problems that are not directly visible as they are hidden beneath the surface of the ground.
A situation occurring repeatedly is when cables get stuck in buried microducts. Time is money, and digging-up is even more money. For obvious reasons everyone would wish for ducts always lying perfectly in the ground. Unfortunately that is not always the way things are in real life.
In those situations, we often see operators trying to force cable and machine beyond maximum to avoid digging-up. Eventually this will usually result in a broken fiber cable. Many factors can influence the situation. Is the cable maybe not stiff enough? Is the compressor too small? Has lubricant been added? And in the right quantity? Or is the blowing machine maybe not good enough?
In nearly all cases, it is the human factor that causes the problem. Not by ill will, of course, but in many places we see the “sins of the past” neatly buried by those who originally installed or relined the duct - or by those who may have done other kinds of work right close to the duct.
The Specific Job
The pictures shown here are from a job in a Danish urban zone stretch in Mid Jutland. The situation is that a 40 mm main duct has been relined with seven 10 mm microducts over a stretch of approx. 6,500 meters, obviously with various connections on the way. The duct arrangement in the wells is fine, but the ducts are not of recent date. Most of the ducts are already occupied.
The technician is to blow a new 6 mm fiber cable into one of the vacant microducts, using a MiniFlow RAPID (bit.ly/21Hjc4s). Air supply comes from a screw compressor with a constant airflow of 1,000 l/min and 15 bar. The first 630 meters go smoothly but then the fiber cable suddenly stops.
Luckily, this is an experienced technician, who stops the machine in time and does not try to force the cable any further, as there is every indication of an error in the microduct. He measures up the stretch of 630 meters and ends up in a peaceful place on flat ground by a roadside with no other visible installations.
The digging-up shows that the contractor, who originally relined the duct, neglected to stagger the connectors while connecting the microduct ends. Having repaired the damage, the remaining blowing process went smoothly and without problems.
Had the technician kept pushing and broken the fiber cable, he would have started an endless series of phone calls between suppliers and customers - it takes time and a lot of money.
The microducts are tight and allow foam sponge and air to pass. Unproblematic on the face of it. Please note that all connectors have been placed right next to each other very close to the main duct. This means that the total diameter of the microducts by far exceeds the inner diameter of the main duct. And in this case the result is two bent ducts.
Had the connectors been staggered by the length of a couple of connectors, the problem would never have occurred.
In this case, the technician had mounted a cable tip with half a millimeter larger diameter than the fiber cable.
The tip became fully wedged in the bend.
The picture is clear.
The inner diameter of the microduct has been reduced by more than one third, and the fiber cable can obviously not get through irrespective of amount of air or the performance of the blowing machine.
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